This invention relates generally to control circuits for electric motors and more particularly to an improved electronic starting control circuit for an alternating current motor.
Single phase alternating current motors normally include a start winding in addition to one or more run windings. The start winding and run winding are generally physically displaced from each other on the stator of the motor and carry currents that are displaced in phase. To start such motors, both windings are connected to a single phase alternating current supply line and, because of the phase displacement between their currents, starting torque is developed. As the motor approaches operating speed, it is known that it is desirable to disconnect the start winding from the supply line in order to operate the motor more efficiently. Additionally, since most start windings are designed to be energized for only very short periods of time, they are not designed to carry current for longer periods of time and failure to disconnect the start winding from the alternating current source would probably result in serious damage to the start winding. Heretofore, the disconnection of the start winding from the alternating current source has commonly been accomplished by use of a mechanical centrifugal switch in the start winding circuit. Such mechanical switches are relatively inexpensive devices but have a life span which is usually substantially shorter than the life span of various electronic switching devices. In order to take advantage of the longer life span of electronic switching devices, voltage and current relays including solid state switches have been utilized to disconnect the start winding from the alternating current source. However, this approach to disconnecting motor start windings has the disadvantage that for different size motors, different circuits must be utilized. As a result, this solution has proved to be uneconomical.
Yet another approach to solving the problem is that illustrated in U.S. Pat. No. 3,882,364 to Floyd H. Wright, et al. entitled "Induction Motor Control System." This patent suggests the use of a sensing coil positioned on one of the stator poles to develop a speed responsive control signal resulting from flux variation, or slot ripple, resulting from the moving rotor teeth. This ripple voltage is then amplified, filtered and compared to a reference voltage to determine when to trigger a triac switch in the start winding circuit to disconnect the start winding from the alternating current source. This approach has several problems associated with it. First, the addition of a sensing winding to the motor pole is a complicated and costly manufacturing step. Second, it is a common practice in the manufacture of induction motors to skew (helically twist) the rotor slots in order to minimize the cogging torque particularly on starting. Such skewing would render the sensing approach suggested in this patent virtually useless because the amplitude of the ripple voltage would be substantially masked by noise. Finally, it is further known that for most rotors it is desirable to close the rotor slots to minimize ripple problems. Since the approach suggested in this patent requires the use of a toothed rotor, such a closed slot construction would render this approach once again relatively useless.